Activated carbon has been widely used in the past for noxious gas adsorption removal, gas purification and separation/recovery, gas occlusion, molecular sieve applications, and for decoloring purification in fields relating to foods or the chemical industry, water treatment, electric double layer condensers and the like, but activated carbon is also commonly used for various filters because of its excellent ability to adsorb and remove a variety of malodorous substances across a wide range. Activated carbon for filters is often used in the form of powder or in pulverized form packed into a container, but it can also be conveniently used in many cases in the form of molded tubes or sheets. In recent years, however, despite high expectations for such activated carbon filters, their uses have been limited because of a high degree of pressure drop.
Many types of filters modified to reduce pressure drop have been proposed to date, such as an adsorption filter comprising an adsorbent, a fine particle binder and reinforcing fibers, disclosed in Japanese Unexamined Patent Publication No. HEI 3-151012. In this filter, the flat filter is made of particulate activated carbon coated with a thermoplastic material such as polyethylene, with packing in a frame set on a polypropylene net, while an example is given of stacking polyethylene nets for air purification.
Japanese Unexamined Patent Publication No. HEI 3-238011 discloses an air purification filter comprising a laminated combination of an electret filter and a flat sheet. This filter reduces pressure drop by employing a corrugated sheet. Also, Japanese Unexamined Patent Publication No. HEI 4-74505 discloses an air purification filter element with low pressure drop by laminating an electret filter and an adsorbent-containing filter into an integrally formed pleat. However, these filters still exhibit high pressure drop of the flat sheet to which the activated carbon is attached, and hence their uses are often restricted.
On the other hand, there is disclosed in Japanese Unexamined Patent Publication No. HEI 5-177133 an activated carbon-adhered fiber sheet wherein powdered activated carbon is adhered to fibers without an adhesive. This sheet is obtained by dispersion, opening and activation of fibers such as glass fibers and a resin powder using an opening cylinder. Also, Japanese Unexamined Patent Publication No. HEI 6-219720 discloses activated carbon with a specified surface acidity, while Japanese Unexamined Patent Publication No. HEI 9-271616 discloses a deodorizing filter material obtained by forming an activated carbon layer on one surface of a fiber sheet and forming an activated inorganic adsorbent layer on the other surface. Also, U.S. Pat. No. 5,124,177 and U.S. Pat. No. 5,338,340 both disclose filters having activated carbon dispersed in an adhesive-coated sheet, where an air jet is sprayed to attach the activated carbon to the sheet.
However, although the techniques disclosed in these publications can provide firm attachment of the activated carbon to the sheet because they both employ pressing with a roll and high-pressure air blowing to attach the activated carbon to the sheet, it is still difficult to avoid a large pressure drop due to external pressing force on the activated carbon.
The present applicant had previously discovered that an easily fabricated flat filter with excellent adsorption performance can be produced by a fluidized adhesion method and has already filed Japanese Patent Publication No. HEI 9-19920 as a patent application therefor (Japanese Unexamined Patent Publication No. HEI 10-204384). This method is a method of manufacturing a filter by attaching an adhesive to a sheet and passing the sheet through a fluidized bed of adsorbent particles, pressing at a pressure roll, shedding the insufficiently adhered particles and drying, and this may be said to be a highly efficient flat filter manufacturing method from the standpoint of production.
According to this method, however, the adsorbent such as activated carbon adheres in a randomly laminated fashion to the adhesive-coated sheet, and therefore the obtained filter does not always have low pressure drop. In addition, although the insufficiently adhered particles are shed by vibration with a vibrator, the activated carbon adhered to the base fabric is sometimes shed during use, even after this shedding.
By utilizing a technique known as electrostatic setting, which is completely different from the manufacturing methods described above, it is possible to obtain an activated carbon sheet with low pressure drop and firm anchoring of activated carbon to the sheet. As prior art adsorbent materials or deodorant materials comprising activated carbon fibers set in a sheet using the electrostatic setting technique, for example, Japanese Examined Patent Publication No. SHO 57-31453 discloses an adsorbent material comprising activated carbon fibers set into a base material, and Japanese Unexamined Utility Model Publication No. HEI 6-85034 discloses a deodorant material comprising activated carbon fibers electrostatically set on the surface of a sheet-like base material precoated with an adhesive on the back side. Both of these methods employ activated carbon fibers as the material, but when the purpose of a filter is considered, since using activated carbon fibers means that a lower amount is used than when using particulate or pulverized activated carbon, the use of particulate or pulverized activated carbon is preferred from the standpoint of functional life.
As an example of the electrostatic setting technique being applied to produce an adsorbent layer using activated carbon as the material, Japanese Unexamined Patent Publication No. SHO 50-144680 discloses a method of forming an adsorbent layer wherein an adsorbent element such as activated carbon is evenly adhered and anchored as an adsorbent layer by electrostatic adhesion. Also, Japanese Unexamined Patent Publication No. SHO 63-274429 describes a composite adhesive sheet wherein a sticking agent layer is provided on a porous sheet material and an adsorbent layer of activated carbon powder is provided on the sticking agent layer by electrostatic setting.
However, the adsorbent layer disclosed in Japanese Unexamined Patent Publication No. SHO 50-144680 uses beady activated carbon, and being in the form of beads it does not always adhere satisfactorily to the sheet, such that a cost-effective product cannot be obtained. The adsorbent sheet disclosed in Japanese Unexamined Patent Publication No. SHO 63-274429 is used as a tobacco filter and therefore employs fine powdered activated carbon, with the binder being printed at a prescribed spacing in the porous sheet and the activated carbon powder being attached to the binder, and therefore the air permeability resistance is significant, as is also mentioned in the examples.
Fiber structures with high-performance permeability are also disclosed in Japanese Unexamined Patent Publication No. HEI 10-102366, Japanese Patent Publication No. 2,818,693 and U.S. Pat. No. 5,486,410. These comprise adsorbent particles of activated carbon or the like with heat-fusible fibers, and are described as having high air permeability. However, the fiber-like structures disclosed in these publications have a structure in which the adsorbent particles are tightly held in the heat-fusible fibers, and are therefore described as having high air permeability, but considering the structure, a large pressure drop is unavoidable.
Low pressure drop is an important function required for filters, but despite numerous attempts to achieve it, no fully satisfactory solution has yet been found. It is therefore an object of the present invention to provide a new filter element with firm adhesion of the adsorbent, without impairment of deodorizing function or adsorption performance, and with low pressure drop, as well as a method for its manufacture and a filter using the element.